240 INVERTEBRATE MORPHOLOGY. 



undoubtedly homologous with the nephridia of the Chsetop- 

 oda, possessing the same relations. In a few forms {Bonellia, 

 PhascoUon) a single nephridium only is present. In addition 

 to these in Echiurus, Thalassema, and allied genera there is a 

 usually much-branched organ on either side lying in the body- 

 cavity and opening into the terminal portion of the intestine. 

 Numerous ciliated funnels occur upon the branches placing 

 the organ in communication with the body-cavity. This so- 

 called " respiratory tree " (so named from a supposed homol- 

 ogy with the similarly named organs of the Holothuria (q. v.) 

 are probably nephridia, though whether or not they per- 

 form excretory functions is not quite clear. In Priapidus 

 these organs are represented by branched tubes, the branches 

 of which terminate blindly in flame-cells, resembling thus the 

 excretory organs of the Platyhelminths, and in SipuTtculus 

 rudiments of these organs have been described as short tubes. 



The Gephyrea are bisexual, the reproductive organs {oc) 

 forming small digitate, elongate, or ovoid processes arising 

 from the peritoneal lining of the body-cavity ; but in some 

 forms (Sipunculus) their products early escape into the coe- 

 lomic cavity, in which they float. The exact manner in which 

 the ova and spermatozoa escape to the exterior has not been 

 definitely ascertained for the majority of forms, but it seems 

 probable that the nephridia serve as the generative ducts. 

 In Friapulus the " respiratory trees " are said to give rise to 

 the reproductive organs, and also to serve as the reproductive 

 ducts — a behavior which would render exceedingly probable 

 the supposition that they are modified nephridia. 



Two orders are recognizable in the Gephyrea. 



1. Order Echiurese. 

 The Echiureae, sometimes known as the Gephyrea armata, 

 are characterized by the presence on the ventral surface of 

 the body, in front of the openings of the nephridia, of a pair 

 of setae — the genus Echiurus possessing, in addition to these, 

 two circles of setae at the posterior extremity of the body. 

 The anus is terminal in all the known species, and the ter- 

 minal portion of the intestine has opening into it the 

 branched respiratory trees. The anterior end of the body is 



